Window Leaks in Calgary: Why Interior Wall Wet Spots Start at the Window Frame, Not the Roof

A Wet Spot on Your Interior Wall After Rain Isn't Always the Roof

When water shows up on an interior wall after a rainstorm, most Calgary homeowners assume the worst — a roof leak, a foundation problem, a burst pipe somewhere inside. Those are real possibilities, but one source gets overlooked more often than it should: the window itself, and specifically the seal between the window frame and the surrounding wall assembly.

Window-related water intrusion shows up in predictable patterns. A wet patch directly below a window sill, discolouration along the interior trim, or damp drywall in the corner of a window rough opening are all signs that water is getting in at the window-wall junction rather than through the wall surface itself. In Calgary's climate — where driving rain combines with significant temperature swings — that junction is under stress year-round in ways that accelerate deterioration faster than most homeowners expect.

How Water Gets Past a Window and Into Your Wall

Water doesn't need a large gap to cause damage. A failed caulk joint 3mm wide is enough to direct water behind the window casing and into the rough opening. From there, it wicks into the framing, travels down the interior face of the exterior sheathing, and shows up as a wet patch on the drywall — sometimes a foot or more below where it actually entered.

There are three main failure points at any window:

The exterior caulk joint between the window frame and the exterior cladding. This is the first line of defence. Silicone and polyurethane caulk shrink, crack, and lose adhesion over time — especially with Calgary's freeze-thaw cycling. A caulk bead that was applied in 2010 may look intact from ten feet away but have hairline separations along most of its length.

The window flashing and sill pan. Proper window installation requires a sill pan — a sloped surface beneath the window that directs any water that gets past the outer caulk line back out to the exterior. In Calgary homes built before the mid-2000s, sill pans were often absent, undersized, or installed without the correct slope. When no sill pan exists, water that breaches the outer caulk joint has nowhere to go except into the rough opening framing.

The interior air seal. Around the inside perimeter of the window frame, the gap between the frame and the rough opening framing is typically filled with low-expansion spray foam or fibrous insulation. If that seal fails or was never properly installed, warm interior air carrying moisture can reach the cold exterior sheathing, condense, and wet the framing from the inside — a separate mechanism from rain intrusion, but one that produces the same damp drywall result.

Why Calgary Specifically Makes This Worse

Calgary averages more than 30 freeze-thaw cycles per year, with temperatures that can drop well below -20°C in January and then climb above zero within the same week [Environment and Climate Change Canada climate normals for Calgary]. Every freeze-thaw cycle expands and contracts the materials around your window frame by different amounts — aluminum frames, vinyl frames, wood framing, and exterior cladding all have different coefficients of thermal expansion. That differential movement works on caulk joints and flashing connections every single year.

The city also sits in a wind corridor that produces driving rain events where water is pushed horizontally against wall surfaces with enough force to find gaps that wouldn't be an issue in calmer conditions. A window that passes a simple drizzle with no leakage can fail during a rain event with sustained wind.

Most Calgary windows 15 years or older have exterior caulk that is overdue for replacement. Windows installed during the late 1990s and early 2000s — a high-volume construction period in Calgary's expanding suburbs — are now at or past the typical 15–20 year caulk service life [NEEDS VERIFICATION — source confirming caulk service life range for Alberta climate conditions].

How to Narrow Down Whether It's the Window

Before calling anyone, run through these checks yourself:

Look at the exterior caulk line. Go outside when it's dry and run your finger along the caulk bead where the window frame meets the exterior siding or stucco. If the caulk pulls away from the surface, cracks when flexed, or has visible gaps at the corners, it needs to be replaced.

Check the interior drywall location relative to the window. If the wet patch is directly below the window sill or in the corner of the window opening, that's consistent with window-related intrusion. If the wet patch is in the middle of the wall with no window nearby, look elsewhere.

Look for staining on the window sill itself. Mineral staining or paint discolouration on the interior wood sill indicates repeated wetting. Even if the sill feels dry now, staining tells you water has been getting in over multiple events.

Check whether the problem correlates with wind direction. If the wet patch only appears after rain events with wind from a specific direction, and that direction faces the same wall as the window, the correlation is significant.

Look at the exterior framing of the window from outside. If you have a ladder and the window is accessible, check whether the top flashing — the piece that sheds water away from the top of the window frame — is properly lapped over the exterior cladding. If the cladding runs over the flashing rather than the reverse, or if there's no flashing at all, water will run directly behind the cladding and into the framing.

What the Fix Actually Involves

If the exterior caulk is the only issue, recaulking is a straightforward job. Old caulk gets removed completely — a process that takes longer than applying new caulk — and new 100% silicone or polyurethane exterior-grade caulk is applied in a continuous bead. Silicone caulk rated for exterior use in climates with freeze-thaw cycling performs better long-term than paintable latex caulk for this application. The cost to have a contractor recaulk the windows on a typical Calgary home ranges from approximately $300–$700 depending on the number of windows and whether flashing gaps are also being addressed [NEEDS VERIFICATION — Calgary-specific recaulking cost source needed].

If the sill pan is absent or failed, the repair is more involved. Proper remediation requires removing the interior drywall around the window, exposing the rough opening, installing or replacing the sill pan with correct slope and end dams, resealing, and redrywalling. Depending on how long water has been getting in, the rough opening framing may also have mould or rot damage that needs to be addressed before closing the wall. In Calgary, mould remediation in wall cavities is regulated under the Alberta Building Code and WorkSafe standards depending on the extent of the affected area [NEEDS VERIFICATION — specific Alberta or City of Calgary reference for residential mould remediation thresholds].

If the interior air seal has failed, the repair is typically done from inside during a renovation: the window casing is removed, old spray foam or insulation is pulled out, and the gap is resealed with appropriate low-expansion spray foam before recasing.

What the Damaged Drywall Repair Involves

Once the water source is fixed, the drywall can be addressed — but sequence matters. Repairing the drywall before fixing the source is money wasted. Water-damaged drywall that has been wet repeatedly is typically replaced rather than repaired. The paper face of drywall deteriorates with moisture exposure, the gypsum core weakens, and mould can colonize both the paper and the cavity behind the board.

For a window-related leak that has been caught relatively early — one or two rain events, wet patch but no active mould — the repair usually involves:

• Cutting out the damaged section of drywall
• Inspecting and drying the framing behind it
• Treating any mould-affected framing with an appropriate encapsulant if the affected area is minor
• Installing new drywall, taping, mudding, and finishing to match the surrounding wall

For a leak that has been ongoing — staining, soft drywall, visible mould, or paint bubbling repeatedly — the scope expands. The full rough opening may need to be opened, framing assessed for structural damage, and the repair treated as a proper water damage remediation rather than a patch.

The cost for drywall repair around a window opening in Calgary varies based on the extent of damage. A straightforward cut-and-patch on a contained wet patch runs roughly $400–$900 [NEEDS VERIFICATION — Calgary drywall repair cost source for 2024–2025 needed]. If the scope includes framing repair or mould remediation, that number increases significantly.

Don't Patch the Wall Without Fixing the Window First

The only reason to mention this directly is that we see it regularly. A homeowner notices a wet patch, patches the drywall, paints over it, and calls it resolved. Six months later, after the next driving rainstorm, the same patch is wet again.

Water intrusion at a window follows the same path every time the conditions repeat. Fixing the drywall without addressing the caulk failure, the missing sill pan, or the failed air seal doesn't change what happens when the next storm hits. It only delays the visible evidence while the water continues to wet the framing behind the wall.

If you've patched the same area more than once and it keeps coming back, the source hasn't been fixed.

Questions about your project? Give Mike a call.

📞 (825) 747-0464 🌐 drybuild.ca